Downloading strangeness: downloaded file has no content

For those requesting that a file be downloaded from my website, having
selected a file (a .zip or a .dmg), the following is then executed:

<font color="#000000">header('Content-Description: File Transfer');</font>
header('Content-Type: application/octet-stream');
header('Content-Disposition: attachment; filename="' . basename ($file) .
'"');
header('Content-Transfer-Encoding: binary');
header('Expires: 0');
header('Cache-Control: no-cache, must-revalidate, post-check=0, pre-check=0');
header('Pragma: public');
header('Content-Length: ' . filesize ($file));

ob_clean ();
flush ();
readfile ($file);


For most of the files to be downloaded, this works just fine. But for one of
them, the downloaded file has zero content at the receiver's end: it is zero
bytes long. The file on the server is correct - I can download it using an FTP
client and drag/drop the file off the server and onto my desktop. Then it has
the correct size/content.

Any suggestions as to why this might be?

-- 
HAL 9000:    Dave.  Put down those Windows disks.  Dave.  DAVE!